Integrated optical devices, such as those formed on a planar lightwave circuit chip, typically include optical components in the form of optical waveguides. In fact, optical waveguides are often the fundamental component of all integrated optical devices. The optical waveguides operate to direct light signals from one location to another, and often branch out or “split” at various locations to allow the light signals to propagate to several different locations.
The spacing between optical waveguides, such as at a splitter location, are ideally made as small as possible. If the spacing is too wide, a larger profile is presented to light signals incident at the splitter location. This wide spacing causes the power incident on the spacing to become non-guided or scattered, and therefore lost in the device. This lost power adversely affects the efficiency and performance of the integrated optical device and the overall performance of the optical network.
Reducing spacing between waveguides is also helpful if chip size is an important consideration. By reducing the spacing between the waveguides, more “real estate” becomes available on the chip for placement of other components, and cost is also reduced.
Reducing space between waveguides, however, is easier said than done due to practical considerations and limitations associated with planar lightwave circuit manufacturing processes. For instance, currently used techniques do not allow for a spacing that is smaller than one given by an aspect ratio (depth divided by width) associated with a particular etch and deposition/re-flow process to form the spacing. The minimum waveguide spacing is determined by the maximum of this aspect ratio—the waveguide spacing cannot be reduced further if the waveguide spacing-to-waveguide height ratio exceeds the maximum aspect ratio allowed by the etch process. Moreover, even if this minimum spacing is reduced via use of an improved etching process, too small a spacing will often leave a void during a subsequent deposition/re-flow process to fill the spacing. This is because the deep and narrow spacing will be difficult to fill completely. The void can cause scattering of light and produces a defect in the circuit.